This invention relates to carburetors for internal combustion engines and in particular to means for producing fuel enrichment to an internal combustion engine during cold start and warm-up periods.
Increasingly stringent motor vehicle exhaust emission regulations, coupled with performance and economy requirements impose increasingly new demands on todays internal combustion components, particularly carburetors.
Difficult problems are encountered during engine warm-up with conventional carburetor systems. Most present day internal combustion engines utilize a carburetor choke system having a choke value pivotally mounted in the air passageway of the carburetor. The choke system is mechanically coupled to a thermostatic coil which, when cold, pushes the choke valve closed. A cold engine is difficult to start. Only the most highly volatile parts of the fuel vaporize, so it becomes necessary to provide extra fuel to the engine manifold in order to insure that each of the individual cylinders receive sufficient fuel for combustion. Conversion of fuel from liquid to a vapor may take place within the combustion chambers of the engine cylinders. When the engine is cranked with the choke valve closed, the choke becomes the air restriction and the entire bore of the carburetor is subject to manifold pressure. Fuel will enter the carburetor from all available passages leading from the bore to the carburetor fuel bowl. Just as soon as the engine starts, the choke valve is partially opened to prevent the manifold from becoming flooded with fuel. A cold engine also requires a richer mixture during the warm-up period, and throughout this period the choke valve is used to restrict the incoming air just enough to produce the extra richness. When the engine reaches normal operating temperature, the choke should be wide open.
It heretofore has been conventional to couple the choke valve to some form of choke break, for example, to a vacuum piston and cylinder, to regulate choke system operation during the warm-up period. In the vacuum piston and cylinder arrangement, manifold pressure is applied to a small piston which rides in a cylinder having two slots or scores which extend longitudinally along the cylinder wall. Full manifold pressure is exerted on the piston until the piston moves into the area having the slots formed therein, after which air is permitted to leak past the piston. Consequently, the choke valve will open under the influence of the piston after the engine starts. The initial opening of the choke valve after start is controlled by the distance the scores extend up the cylinder wall. Air which passes around the piston comes from within the thermostatic coil housing. As indicated, the thermostatic coil also is coupled to the choke valve. The housing is connected to a heat pocket in the exhaust manifold. In this way, the thermostatic coil position becomes a direct indicator of engine temperature. As the engine warms up, the thermostatic coil tension is lessened, and the choke valve is opened accordingly. During acceleration in the warm-up period, opening the throttle raises the manifold pressure, i.e., decreases manifold vacuum. The vacuum piston becomes less effective at holding the choke open. The thermostatic coil tries to pull the choke valve towards the closed position. The difference in pressure above and below the choke valve forces the choke valve open to the proper position for enriched accelerating mixture.
While conventional choke systems work well for their intended purposs, they are not subject to precision regulation. The amount of fuel delivered to the engine is proportional to the flow of air. These proportions are determined by the relative sizes of the air and fuel restrictions. Conventional choke systems vary the air restrictions during start and engine warm-up to provide the necessary air-fuel ratios. With the advent of vehicle exhaust emission regulations, fuel-air mixture enrichment sufficient to operate the engine in cold start situations is difficult to achieve with the conventional choke circuit described above.
The prior art reveals various solutions to the fuel enrichment problem in the present day emission control environment. One such problem is shown and described in the U.S. Pat. No. 3,739,760, to Charron, issued July 19, 1973. While the Charron patent works for its intended purposes, the invention described hereinafter, besides being structurally distinct from the Charron disclosure is an advancement over it in that precise metering of the fuel input to the auxiliary fuel enrichment system may be obtained while the remaining parts of a conventional carburetor structure are retained, including fast idle feature found in present carburetor systems. Because relatively little modification to present carburetor structures is required, the auxiliary fuel system disclosed hereinafter may be applied to a wide range of carburetor models presently utilized with internal combustion engines. The system is designed so that although full manifold vacuum is applied to the auxiliary fuel outlet port, a relatively constant vacuum signal to the metering points of the auxiliary fuel system is maintained by a variable air bleed. Consequently, fuel restriction size is the only other variable in the system and it is controlled by a metering device.
One of the objects of this invention is to provide an auxiliary fuel enrichment system which replaces the conventional choke system in a carburetor structure for an internal combustion engine.
Another object of this invention is to provide an auxiliary fuel system for an internal combustion engine which may be continually monitored and controlled precisely.
Another object of this invention is to provide an auxiliary fuel system compatible with a wide range of carburetor types.
Another object of this invention is to provide an auxiliary fuel system having its fuel input located downstream from or, during certain running conditions, in ported relationship to the throttle plate of a carburetor structure.
Another object of this invention is to provide an auxiliary fuel system having a metered fuel output.
Other objects of this invention will become apparent to those skilled in the art in light of the following description and accompany drawings.